Forces
Sitting Where You Are At this exact moment you are most likely sitting on a chair in front of a desk, staring at your computer screen. Perhaps from your comfortable seat you may have a view of the street via a window. Outside the world is bustling. People might be walking on the street, carrying bags of groceries or pushing baby carts for a stroll. It is possible that raindrops are falling to the ground from cloudy skies, drenching passerbies. All of these sights, from the rain falling to the ground, to the pushing of baby carts, and yes even to you sitting on the chair, are examples of forces which govern the physical universe. Though these interactions may seem obvious, they are in fact examples of force. What Is Force? Force is quite simply an influence or agent acting on an object that induces it to accelerate. An object, known in physics lingo as a system, is affected by the sum of the forces acting upon it in the environment, or the world around the object. Force is a vector quantity, meaning that both the magnitude and direction of a force can be measured. This is helpful when one estimates how much force should be put into throwing a baseball, and of course to whom it is thrown. A force can be exerted by such agents as a push or pull, the masses of objects, and even between the charges of electrons in atoms. Most commonly, forces can be classified into two overarching groups: contact forces and action-at-a-distance forces. ---- Defining Force Units Force is most commonly measured in the SI unit called Newtons or simply abbreviated as N. In case you are wondering, the name does come from the surname of the famous Sir Isaac Newton, who will be discussed later. If an acceleration of 1 m/s2 is applied to an object whose mass is 1 kilogram, the equation will yield a value of 1 m*kg/s2. This unit is equivalent to 1 Newton. In addition the SI system of measurement, the CGS system measures force in Dynes. A Dyne is defined as the force required to accelerate a mass of one gram at the rate of 1 centimeter per second square. So no matter if you are English or American, you have a choice of defining force in Newtons or Dynes. However, since the majority of the world uses SI units, this article will stick to Newtons. Balanced and Unbalanced Forces Because force is a vector quantity, its can easily be represented using Force Diagrams. When identifying forces, one should remember to label horizontal as well as vertical forces. Force can be either balanced or unbalanced. In a balanced force, there are two opposing forces of equal magnitude that essentially cancel each other out. Due to this the object will stay at rest. In an unbalanced force, on the other hand, a force causes motion and according to Newton's First Law, will continue to stay in motion unless another force interferes. On earth, objects that have a force applied unto them will usually not keep on going forever, because the force of friction will eventually bring the object to a stop. An example of an unbalanced force that will cause the object to continue moving as described by Newton’s 1st law, is a hockey puck on an air-table with no friction. In the above diagram, both horizontal and vertical forces are labeled. There is no motion in the vertical aspect, because the force of gravity and the normal force balanced each other out. In the horizontal aspect, on the other hand, the applied force of the hand pushing the book to the left, overcomes the magnitutde of the force of Friction, hence the book will move leftward. Force in Terms of Acceleration As was mentioned beforehand, acceleration on objects occurs as a result of force. This acceleration can be explained by Newton’s Second Law of Motion. This law states that the acceleration of an object is directly proportional to the magnitude of the force and conversely is inversely proportional to the mass of the object. These two principles are illustrated by the following universal equations which have been proved time and time again. Both equations involve mass and Acceleration, which is necessary to calculate force. Although different types of forces require varying equations, they all include mass and acceleration. This is a fundamental rule for force. Force in Terms of Momentum Thus far we have defined force in terms of acceleration. However when Newton first developed his theorems concerning motion and force, he did so in terms of momentum. Newton observed that when an unbalanced force acts on an object, the difference or change in momentum for that object can be expressed as the force multiplied by the time taken for that change in momentum to occur. Taken from honolulu.hawaii.edu To understand the relationahip between force and momentum use the analogy of a baseball game. The pitcher throws the ball towards the batter, who wants to hit a grand slam. In order for the ball to change direction and return from the path from which it was pitched (and hopefully out of the park) the batter must hit the ball with a large enough force, in a small period of time, to cause a significant change in momentum and result in the ball flying out of he park. If the batter does not use a significant magnitude of force, namely by keeping the bat still or "bunting" it, the ball will not fly off at a large speed. This idea is captured in the Impulse-Momentum Theorem. Types of Forces As was mentioned before Forces are generally classified into two groups: Action At a Distance Forces and Contact Forces. In each group, different forces exist, which can be calcultaed through different equations. Since this is a general page on the concept of force, I shall not delve into the specifics of each type of force. Action At a Distance Forces Distance Forces exist without contact. Perhaps the most well-known of these forces is gravity. A gravitational force, known as the Law of Universal Gravitation, exists between all objects, no matter how large or miniscule they are. The earth in particular, with its large mass, causes a gravitational force that keeps us from floating into the atmosphere. The effect of gravity on objects is known as Weight. Notice that when you drop a ball, it fall in a direct line towards the ground. This is because gravity exerts a downward force the pulls accelerates the ball towards the center of the earth, untill the ground stops the ball in what is known as the normal force (see section on balanced force). As was mentioned beforehand every body attracts every other body, regardless of material or size. This kind of attraction can be seen not only by gravitational force, but also by Strong Nuclear Force, weak forces, Electromagnetic force, as examplified by the properties of Electromagnets, and centripetal force as explained by the concept of Centripetal acceleration. Contact Forces As alluded to in the title, contact forces simply involve contact of some sort. Contact force can involve pulling or pushing. Throughout history, man has utiizled contact force to overcome forces such as gravity and the force of friction through inventions such as Pulleys and Levers. ---- An important idea that relates to contact force is shown in Isaac Newton's Third Law of Motion that states that for every action there is an equal and opposite reaction. For example, if two astronauts are floating near their shuttle in outer space, and one astronaut decides that he doesent like his fellow space-voyager, and decides to push him into oblivion, he is in fact making a grave error. By pushing the poor atronaut, an equal but opposite force pushes the vile astronaut in the oppsite direction, so both astronauts fly off into oblivion. A Brief History of Force An age-old cliche is the image of Isaac Newton being hit on the head with an apple in the English town of Woolsthorpe, and then coming up with the idea of gravity. In reality, Newton's contribution to the understanding of the force of gravity took nearly 20 years. In addition, his theorems built upon the ideas of scientists such as Galileo, Descartes, Kepler and Copernicus. Newtons main works are noted in the Prinicipia. Newton based many of his works on gravitation through observation of the relationship between earth and the moon. To do so, Newton based much of his experiments on calculations doen by Kepler. to learn more about Kepler visit Kepler's NASA Site. Summary Questions Because force is a big part of regents test, it is important to realize the overarching idea behind force. Answer the following questions to test your understanding of the topic: 1. What is force? 2. What are balanced and unbalanced forces? 3. Draw a force diagram of an elevator going up 4. Draw a force diagram of a man pushing a lawnmower 5. What are the differences between Contact Forces and Action-at-a-Distance Forces? Resources and References References 1. http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html - A Great Reference for Newton's Three Laws 2. http://science.howstuffworks.com/fpte2.htm - A succinct summation of force 3. http://www.physicsclassroom.com/Class/newtlaws/U2L2a.html - Good images 4. http://encarta.msn.com/encyclopedia_761555718/Force.html - A comprehensive site complete with links 5. http://www.newton.cam.ac.uk/newtlife.html - Information on Sir Isaac Newtons Resources 1. Encyclopedia Britannica - Suggested for those who can deal with headaches, but have a thirst for physics 2. Encyclopedia Americana - Provides an excellent source on the biographies of scientists 3. Barrons Physics Review Book by Miriam A. Lazar - Complete with sample problems 4. Glencoe Physics- Principles and Problems by Paul Zitzewitz - Gives a great definition of Force 5. The Elegant Universe by Brian Greene - Has a very readable section on force (and string theory is also very interesting)